Hi Himansu,
I've assigned the portal desktop to end user just like you did. There is no difference in that.
The final option is to check the theme of HTML DOM Properties during runtime. As ESS Overview screen is a ABAP application it calls the R3 theme during runtime.
Though I've assigned MCL theme in portal, during runtime ABAP applications are fetching SAP Tradeshow plus theme from backend (R3).
To verify the theme in your case, Start F12 developer tools and inspect the Employee Services header. As indicated in the below screenshot, just place the cursor on css link so that it will show the entire css R3 link which contains theme name.
Regards,
Kamal